The present invention relates generally to portable electronic devices and, more particularly, to thermal management in such devices.
Portable electronic devices, such as portable computers, cellular telephones, etc., are becoming more widespread as the convenience and utility of such devices become more apparent. Along with the increasingly widespread use of such devices, new capabilities and functionalities of those devices are being developed that require higher processing capacity. In order to make an electronic device more powerful (to possess greater processing capacity), additional transistors within components of those devices are usually necessary to accommodate more computational operations in a given amount of time. Alternatively, the frequency of calculations made by the transistors may be increased to produce greater computational operations. In portable electronic devices, the number of transistors may be increased by adding additional electronic components, or by increasing the number of transistors on an existing component. Adding components in such devices is usually not desired because doing so requires more circuit board space and, accordingly, a larger device. Therefore, increasing the processing power in such portable devices is typically accomplished by 1) adding transistors to existing components, 2) by increasing the computational frequency of existing transistors, or 3) by utilizing a combination of both approaches.
Heat dissipation is of paramount importance when designing increasingly powerful electronic devices. This is because increasing the frequency or increasing the number of transistors can lead to more heat generated by the electronic component. This concern is especially paramount in portable devices because, typically, the electronic components are contained within a relatively small, confined housing that makes adequate cooling difficult to achieve.
Various attempts have been made to address heat dissipation problems in portable electronic devices. For instance, in portable computers, various configurations of heat sinks, fans and other devices are traditionally used. In some applications, a heat sink is connected to a heat-generating component, such as a central processing unit (CPU) or a graphics processing unit (GPU) disposed on a circuit board within the computer. Heat sinks operate based on the principle that heat is dissipated at a lower temperature over a large surface area as compared to that dissipated over a relatively small surface area. The heat sink, therefore, has a physical geometry (e.g., heat conducting fins disposed on a heat conducting substrate), that creates a large total surface area. Thus, when heat is transferred from the CPU or GPU to the heat sink, heat is dissipated at a lower temperature due to the large fin surface area relative to the case where no heat sink is used.
Fans have also been used, either alone or in conjunction with a heat sink, to provide airflow over the heat-generating component/heat sink. Heat is transferred to the air as it flows over the component, thus cooling the component. However, as greater processing power has been introduced to components in portable electronic devices, such as the exemplary CPUs and GPUs in portable computers, fan-based and heat-sink based cooling solutions have proven undesirable in some instances, either because of insufficient cooling or because of increased noise due to an excessively high fan rotation speed needed to cool the heat-generating component. Therefore, more recently, other attempts have been made to cool these components.
In one such attempt, liquid cooling has been used to remove heat from heat-generating components. In these attempts, a reservoir of liquid (e.g., water) is used that is relatively cool compared to the heat-generating component. Hollow tubes are used with a small pump to transport the liquid to a heat spreader that is connected to the component. When the component is heated, this heat is transferred to the liquid within the heat spreader. The pump causes the water to circulate back to the reservoir where the heat is dissipated. As is obvious, the cooler the temperature of the liquid in the water reservoir, the more heat that can be dissipated. This cooling method has achieved significant reductions in the temperatures of heat-generating components in computers, especially in desktop-based systems. However, this method is less useful in portable electronic devices, such as portable computers because, while liquid cooling can achieve significant heat dissipation, such systems tend to greatly increase the size and weight of the computer if the reservoir is internal to the computer. Additionally, these active pumps may also add cost and decrease the reliability of the cooling system and, hence, the portable device itself. In the case of a portable computer, any such reservoir will be necessarily limited to a relatively small size and, therefore, will only be of limited effectiveness in cooling components within the computer. While an external reservoir could be used, portable computers generally are not manufactured with the necessary components to interface a heat-generating component with such an external reservoir. Since disassembling a portable computer can be technically challenging (compared for example to a desktop computer), such an interface would be difficult to achieve.
In another recent attempt to cool portable computers, phase-change devices have been used. Such devices take advantage of the fact that it takes a significant amount of thermal energy to change a substance from one physical state to another. These devices can utilize either a liquid-gas phase change or a solid-liquid phase change.
One example of a liquid-gas type of phase-change device is a device known as a xe2x80x9cheat pipe.xe2x80x9d A heat pipe typically uses one or more hollow tubes (or pipes) to transport heat from one point to another (e.g., from a heat-generating component to a cooler location). Such devices usually are connected to a heat spreader device that is connected to the heat-generating component. The hollow tubes are used to transfer the heat from the heat spreader to a heat sink located at a cooler location (e.g., near a vent in a computer). The hollow tubes are typically filled with a fluid, such as distilled water. Typically, the liquid is at a very low pressure because the tube itself contains very little fluid in order to reduce the evaporation point of the liquid. In the portion of the heat pipe that is in contact with the heat spreader, the heat from the heat-generating component causes the water to evaporate. The vaporized water transports the heat through the hollow tube(s) to the aforementioned heat sink. Once in the heat sink, the evaporated water cools, condenses and returns to the heat spreader as a liquid via the hollow tubes. The same amount of heat is removed as in previous attempts however, because of the evaporation/condensation process, far less temperature difference between the heat generation component and the heat sink is required.
However, such heat pipe devices are limited in their usefulness. Specifically, such devices cannot transfer the amount of heat generated by many kinds of components, such as state of the art CPUs. Thus, this solution may be insufficient to cool components with an extremely large number of transistors or transistors that operate at a very high frequency. Additionally, the heat sink of such heat-pipe devices must usually be in a higher position than the heat spreader. With portable computers this may be problematic as the geometric form of such computers is such that it may not be possible to place the heat sink in this position.
Another example of a liquid-gas phase change device used with computers is a compressor- or refrigerator-based device. These devices work much the same as the heat pipe type system, discussed above. Similar to a heat pipe device, refrigerator-based systems remove heat from a heat-generating component with a heat spreader connected to hollow tubes. With refrigerator-based systems, however, the tubes are filled with a refrigerant substance that is cooled by a refrigerator to the point where it changes to a liquid state. The liquid refrigerant is then passed through the hollow tubes to the heat spreader that is in contact with the heat-generating electronic component. Since the refrigerant liquid is at a relatively low temperature, significant thermal energy is absorbed by the liquid in order to heat it to the temperature at which it evaporates and, subsequently vaporize the liquid, thus removing significant heat from the component via the heat spreader. The gaseous substance is then passed once again to a heat sink (condenser) where heat is removed from the refrigerant at which point it is converted back into a liquid.
Refrigerator-based systems are advantageous in that they can remove significant heat from a component. In fact, such components can be cooled below the ambient temperature using this method. However, such cooling is limited in its practical usefulness. Importantly, the extreme temperature difference between the refrigerant and the surrounding atmosphere can result in significant condensation forming on the external surfaces of the heat spreader and hollow tubes. Since these tubes are often disposed within an electronics enclosure (such as a computer case), such condensation is highly undesirable as it can damage the electronics components therein. Additionally, as in liquid-based systems, refrigerator-based heat-dissipation devices are often large and heavy and must interface with components within the computer. Also, electrical work must be supplied to the refrigerator device and dissipated as heat. Therefore, such systems are typically impractical for use with portable devices, such as portable computers.
In yet another previous attempt, a phase-change device based on changing the state of a substance from a solid to a liquid has been used to store heat and therefore, for example, to dramatically increase the time for the external housing of the computer to become hot. Such solid-liquid devices typically use a solid substance, such as wax, contained within a reservoir. The reservoir is disposed in a position such that heat is transferred from a heat-generating component to the reservoir and, hence, the wax. The heat/thermal energy is transferred to the wax, thereby causing the wax to melt. As discussed above, such a phase change requires a significant amount of energy and, therefore, such devices possess acceptable heat-dissipation properties for some uses. Additionally, such devices are usually small and reusable. However, wax only requires approximately one-tenth the energy per unit mass to change from a solid to a liquid as compared to water changing from a liquid to a gas (e.g., in the heat pipe example or the condenser-based example). Therefore, less heat (thermal energy) is removed from the heat-generating component using such a wax-based system. Additionally, once all the wax in the cartridge has been liquefied, the device becomes generally ineffective at removing heat. In this case, the wax cartridge must be cooled so that the wax can re-solidify before being able to be used to remove significant levels of heat. In many cases this can be impractical as significant time may be required for such cooling. In addition, several interchangeable, relatively heavy wax reservoirs may be required to maintain effective cooling.
The present inventors have realized that, while the above prior attempts are advantageous in some regards, each of these attempts is limited in at least one aspect of its usefulness in dissipating heat in electronics devices and, more particularly, portable electronic devices. In particular, none of the previously discussed attempts are 1) capable of dissipating sufficient heat from the device; and 2) lightweight/easily portable; and 3) easily rechargeable such that heat dissipation is not interrupted for an extended period of time.
The present inventors have developed a heat dissipation device for use in an electronic device. This heat dissipation device includes a reservoir for holding a substance in a liquid state and at least one heat transfer point for transferring heat from a heat source, such as a component in the electronic device, to the liquid. This heat causes the substance to transform into a gaseous state due to the transfer of thermal energy from the heat source to the substance. A gas permeable membrane may be used to permit the substance to escape the reservoir when it is in a gaseous state while, at the same time, preventing the substance from escaping when it is in a liquid state. In one embodiment, the aforementioned reservoir may be a cartridge filled with ordinary tap water that is inserted into a portable electronic device, such as a portable computer. When this reservoir is inserted into the device, it directly or indirectly contacts a heat-generating component within the device. Heat is thus transferred from the component to the water, thereby changing the state of the water from a liquid to a gaseous state. This gas is then allowed to escape the reservoir via a gas permeable membrane thereby dissipating the thermal energy of the gas to the atmosphere.